The present invention relates to a diazotype heat development recording medium wherein a recorded image may be fixed, that is, a non-image portion will not cause color development again.
Various conventional imaging methods for converting information into a visual image are known. In general, an image is generally formed by causing a physical or chemical change in response to energy such as light, radiation, electrolysis, magnetism, heat, or pressure.
Heat sensitive recording methods are roughly classified into two types; one utilizes a physical change such as melting, sublimation, or volatilization and the other employs a chemical change by heat. However, a recording medium adopting either of these methods records an image using heat. Therefore, even after recording a non-image portion still retains color development capacity upon application of heat. When the recording medium with an image recorded thereon is brought close to a heat source, the non-image portion is developed, thus impairing the recording function of the medium. Improvements in this respect are highly desirable.
A diazotype heat development recording medium as a dry imaging system has recently been receiving a lot of attention as a heat sensitive recording medium, and extensive studies are being made on it.
The diazotype heat development recording media are classified into three types: alkali precursor type, coupler precursor type, and diazo precursor type. These three image forming methods adopt basically the same principle. First, a diazo compound and a coupler cause coupling reaction by heat energy to form an image with an azo dye. Then, the entire surface of the medium is radiated with light energy to cancel the color development capacity of the non-image portion of the medium and to form a permanent image. However, the three methods differ from each other in the method for causing the coupling reaction. More specifically, in a heat development recording medium of the alkali precursor type, a thermal developer (alkali generating agent) causes a physical change such as decomposition, dissociation, or melting by thermal energy to place the recording layer in an alkali atmosphere. A diazo compound and a coupler then cause the coupling reaction to form an image with an azo dye. In a diazotype heat development recording medium of the coupler precursor type, a coupler which may not cause the coupling reaction at normal temperature is activated by thermal energy to cause the coupling reaction with a diazo compound and to form an image with an azo dye. A heat development recording medium of diazo precursor type utilizes structural isomerization of diazosulfonate by light. An anti diazosulfonate which does not cause the coupling reaction by light energy is isomerized into a syn diasosulfonate which causes the coupling reaction with a coupler upon application of thermal energy, thereby allowing formation of an image with an azo dye.
Diazotype heat development recording media of the three types described above are already known and many patents on them have already been granted. For example, diazotype heat development recording media of the alkali precursor type which use various development assistants are known, as per U.S. Pat. No. 2,653,091, and Japanese Patent Publication Nos. 45-8500, 43-10248 and 49-3926.
Diazotype heat development recording media of the coupler precursor type are known as per Japanese Patent Publications Nos. 45-40153, 47-11797, 49-1562, and 50-14522. These recording media use a 2,3-dihydroxy benzoic acid derivative as a coupler which may cause the coupling reaction by thermal energy.
Diazotype heat development recording media of the diazo precursor type and recording methods utilizing the same are known as per U.S. Pat. No. 2,217,189, U.K. Pat. No. 544,702, DE-AS No. 734,302, and Japanese Patent Disclosure No. 56-5790. These diazotype heat development recording media respectively consist of a diazosulfonate compound, a coupler and a polymeric binder.
However, all these diazotype heat development thermal recording media are subject to disadvantages. For example, diazotype heat development recording media of the alkali precursor type have poor storage stability although they have good heat sensitivity. Diazotype heat development recording media of the coupler precursor type and diazo precursor type have poor heat sensitivity although they have good shelf life. Although extensive studies are being made with a view to solving this problem, a diazotype heat development recording medium which has both good heat sensitivity and shelf life has not yet been proposed.
As a result of experiments conducted by the present inventors on the conventional diazotype heat development recording media, it has been found that heat sensitivity depends on the recording material. Thus, the sensitivity of a diazotype heat development recording medium of the coupler precursor type or diazo precursor type is dependent on the heat sensitivity of the coupler and the diazosulfonate. The heat sensitivity of these organic compounds is determined, in turn, by the position and type of the substituting group. However, when a substituting group for improving heat sensitivity is added, only a slight improvement is obtained. The low heat sensitivity of the recording media of these two types is also attributable to the structure of the recording material. For this reason, the recording media of these types are not suitable in practice and were removed from consideration.
A diazotype heat development recording medium of the alkali precursor type has a recording layer which contains as basic recording elements a diazo compound, a coupler, an acid stabilizer, and a thermal developer and which is formed on a support. The recording layer contains a thermal developer for generating a thermal coupling atmosphere and is basically the same as a commercially avaialble dry or wet type diazotype photosensitive paper except for this thermal developer. Therefore, the shelf life and heat sensitivity of the recording medium largely depend upon the characteristics of the thermal developer used; i.e., chemical or physical stability under normal temperature, solubility in water, thermal coupling capacity and the like. For this reason, the thermal developer to be used in the diazotype recording medium must satisfy the following requirements: chemical and physical stability at temperatures below 50.degree. C. for long shelf life, no solubility in water, and ability to immediately generate a coupling atmosphere at a temperature of 100.degree. to 150.degree. C. for good heat sensitivity, and the like.